Chemical mechanical grinding system and method for hard and brittle semiconductor wafers

Abstract

The invention relates to a chemical mechanical grinding system and method for hard and brittle semiconductor wafers. The chemical mechanical grinding method comprises the following steps: using an organic thermal decomposition type oxidant and an organic photodecomposition type oxidant, polyethylene glycol, glycerol and vegetable oil to prepare an oil-based thermally active-photoactive composite grinding fluid; constructing a chemical mechanical grinding platform for an ultraviolet light-diamond abrasive grinding wheel with an ultraviolet light irradiation function and adjustable and controllable irradiation intensity and ultraviolet light wavelength; starting the chemical mechanical grinding platform and a minimal lubrication liquid supply device to control the wafer to be processed and the diamond abrasive grinding wheel to rotate and grind each other; turning on an ultraviolet light generator to precisely irradiate droplets of the thermally active-photoactive composite grinding fluid on the wafer surface, utilizing grinding heat energy and ultraviolet light irradiation energy to induce the thermally decomposition type and photodecomposition type oxidants to generate active free radicals, triggering a free radical oxidation reaction in the wafer surface material, and forming an oxide layer with low interfacial bonding strength; and removing the oxide layer on the wafer surface by micro-cutting with a small load of diamond abrasive, thereby efficiently obtaining a semiconductor wafer with a subsurface nearly free of mechanical damage.

Description

Technical Field

[0001] The present invention relates to the technical field of ultra-precision processing of semiconductor wafers, and in particular to a chemical mechanical grinding method for hard and brittle semiconductor wafers. Background Art

[0002] Semiconductor materials, such as single crystal silicon, silicon carbide, and gallium nitride, are core materials in the fields of energy, communications, transportation, and medicine. Power devices and radio frequency devices made of semiconductor materials are widely used in modern industrial fields such as new energy vehicles, 5G communications, photovoltaic power generation, rail transportation, smart grids, and aerospace. However, semiconductor materials generally have extremely high hardness, extremely low fracture toughness, extremely strong wear resistance, and stable chemical properties, making them a typical type of difficult-to-process material. The surface quality and processing accuracy of semiconductor wafers will significantly affect the service performance, energy consumption, and life of power and radio frequency devices. Therefore, there is an urgent need to achieve high-efficiency, low-damage, and ultra-precision processing of semiconductor wafers.

[0003] At present, both domestic and foreign methods mainly use grinding and polishing to perform ultra-precision processing on semiconductor wafers cut by wire sawing to control shape accuracy, surface quality, and subsurface damage depth. Grinding usually uses 1-15μm free diamond abrasives to mechanically remove the damage layer generated by the wire saw cutting process. CN117645841A and CN117447963A provide a method for mechanically grinding hard and brittle silicon carbide wafers with free abrasives. However, the material properties of high hardness and strong wear resistance, as well as the processing method of removing materials by three-body wear, result in an extremely low material removal rate and poor workpiece surface accuracy when grinding silicon carbide with free abrasives. In view of the material properties of high hardness, strong wear resistance, and high brittleness of third-generation semiconductor materials, existing grinding methods for processing hard and brittle semiconductor materials are more or less faced with the problems of low efficiency or poor processing accuracy. It is urgent to develop an ultra-precision grinding method that can balance processing quality and processing efficiency, reduce the manufacturing and processing costs of the entire brittle semiconductor material, especially hard and brittle semiconductor materials, and accelerate the development of high-performance semiconductor devices.

[0004] Patent CN 116038551 A discloses a photochemical mechanical polishing method that uses ultraviolet radiation energy to induce the decomposition of a photoinitiator compound in an aqueous photosensitive polishing fluid to produce benzoyl radicals and alkyl radical active species, which chemically modify the workpiece under mechanical force, inducing the formation of a modified layer with low hardness, small elastic modulus and high fracture toughness on the workpiece surface. Finally, the modified layer is ground with a grinding disc to increase the critical cutting depth for brittle-to-plastic transition removal of the material, reduce or even eliminate processing damage caused by single mechanical stress removal, and improve material removal rate and high processing efficiency. However, the photochemical mechanical polishing method proposed in this patent has the following three limitations: (1) Due to the inherent limitations of the polishing equipment, the photochemical mechanical polishing method proposed in this patent is only applicable to the surface flattening processing of materials, that is, it can only be used for plane processing, and cannot process curved surfaces and complex microstructures and surfaces; (2) The photoactive polishing fluid involved in this patent is an aqueous polishing fluid using deionized water as a base fluid. Although its excellent cooling performance is suitable for grinding, its weak lubricity leads to severe grinding of the ultrafine abrasive grinding wheel during grinding, requiring repeated dressing of the grinding wheel, reducing processing accuracy and efficiency. At the same time, the aqueous polishing fluid has certain corrosiveness, which will significantly affect the high speed of the grinder. The life and accuracy of the air-floating spindle are shortened, so it is useless for ultra-precision grinding of semiconductor wafers; (3) The temperature of the grinding interface during grinding is usually at room temperature. Based on this, the photoactive grinding fluid proposed in this patent is a solution that is only sensitive to ultraviolet light. It uses the energy of ultraviolet light irradiation to induce the solution to produce benzoyl radicals and alkyl radicals, so it can be used well for grinding. However, this solution cannot use thermal energy to decompose and produce free radicals, and grinding will generate a lot of grinding heat, the temperature can be as high as 200°C or even higher, and excessive grinding heat will cause serious thermal damage. Therefore, it is necessary to invent a thermally active grinding fluid that can absorb grinding heat for grinding.

[0005] Patent CN110842761A discloses a photocatalytic high-energy field-assisted chemical-mechanical composite micro-grinding method for composite micro-grinding of tiny parts made of silicon-based materials. The method is characterized by: supplying an alkaline chemical modification liquid with deionized water as the base liquid to the surface of the silicon-based material part, first using an ultraviolet laser to scan along a preset path, and under the catalysis of ultraviolet light, the material in the scanned area is photocatalytically oxidized and modified into an oxide layer; then switching to an infrared laser to scan again along the original path, the oxide layer continues to be chemically modified into a silicate layer under the assistance of a high-energy field; turning off the laser and starting the micro-grinding tool, which mechanically removes the silicate layer along the laser scanning path; recirculating the above ultraviolet-infrared-chemical-mechanical composite micro-grinding process until the processing size requirements are met; finally, switching to supplying a weak acidic detergent for rinsing, neutralization, and recovery of the residual liquid to complete the processing. However, the photocatalytic high-energy field assisted chemical mechanical composite micro-grinding method proposed in the patent has the following three limitations: (1) Due to the limitation of the laser spot size (the spot diameter range is 10 to 500 microns, less than 1 mm), the size of the oxide layer produced by the collaborative modification of ultraviolet laser and infrared laser is also less than 1 mm. Therefore, the diameter of the micro-grinding rod of the invention patent can only be less than 1 mm, usually in the range of 0.2 to 1 mm. As a result, the grinding method proposed in the patent is only applicable to the processing of micro-parts and cannot be used for high-efficiency ultra-precision processing of large-sized parts such as semiconductor wafers (diameter 120 to 200 mm); (2) The alkaline chemical modification liquid involved in the patent is an alkaline grinding liquid using deionized water as the base liquid. Although the water content can be reduced by adding rust inhibitors, the water content of the alkaline chemical modification liquid is less than 1 mm. The grinding fluid can cause rust on the spindle, but the weak alkalinity generated by the sodium hydroxide in the solution will corrode the machine tool body. At the same time, the weak lubricity of the aqueous grinding fluid leads to severe grinding of the ultrafine abrasive grinding wheel during grinding, requiring repeated dressing of the grinding wheel, which reduces the processing accuracy and efficiency. Therefore, it cannot be used for ultra-precision grinding of hard and brittle semiconductor wafers; (3) The alkaline chemical modification fluid provided by the patent utilizes the inorganic oxidant hydrogen peroxide (H2O2) in the solution to absorb ultraviolet laser energy to generate hydroxyl radicals for oxidation. However, the inorganic oxidant cannot be dissolved in oily solvents such as vegetable oil, so it is impossible to prepare a grinding fluid with both lubrication and oxidation properties. At the same time, the solution only produces a single carboxyl radical, which often reacts with the dissolved oxygen in the alkaline solution to produce an annihilation reaction, resulting in the failure of the oxidation activity. In view of this, the invention patent proposes further high-energy infrared laser processing of the pre-processing area to obtain the required modified layer. However, the high-energy beam generated by the infrared laser can easily cause thermal damage and thermal cracks in the non-processing area. For the processing of semiconductor wafers, once thermal cracks occur, ultra-thin wafers (thickness less than 0.5 mm) will be fragmented. Therefore, the photocatalytic high-energy field assisted chemical mechanical composite micro-grinding method cannot be used for high-efficiency ultra-precision processing of hard and brittle semiconductor wafers. Summary of the Invention

[0006] In response to the prominent contradiction between the grinding efficiency and surface quality of difficult-to-process hard and brittle semiconductor wafers, the present invention provides an ultra-precision chemical mechanical grinding method and a thermally active-photoactive composite grinding fluid. By configuring an oil-based thermally active-photoactive composite grinding fluid, a UV-diamond abrasive grinding wheel chemical mechanical grinding system with UV irradiation function and adjustable and controllable irradiation intensity and UV wavelength is constructed. The grinding heat energy and UV irradiation energy are used to induce organic thermal decomposition and photodecomposition oxidants to produce active free radicals to oxidize the surface material of the semiconductor wafer under mechanical stress, thereby triggering the formation of an oxide layer with low hardness and low interface bonding strength on the wafer surface. The diamond abrasive uses small load micro-cutting to efficiently remove the oxide layer on the wafer surface, reducing or even eliminating the processing damage caused by the high mechanical stress removal of single mechanical grinding, while improving the material removal rate and processing quality.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] In the first aspect, the present invention provides a chemical mechanical grinding system for hard and brittle semiconductor wafers, including a machine tool body, on which a first linear drive device and a second linear drive device are arranged, the first linear drive device realizes the movement of the first high-speed air-floating spindle in the X direction, and the second linear drive device realizes the movement of the second high-speed air-floating spindle in the Z direction, the first high-speed air-floating spindle drives the semiconductor wafer to rotate; the second high-speed air-floating spindle drives the diamond abrasive grinding wheel to rotate; an ultraviolet light generator and a minimal lubrication device are arranged on the outside of the semiconductor wafer; the minimal lubrication device atomizes an oil-based thermally active-photoactive composite grinding fluid into small droplets according to a certain amount and then sprays them onto the semiconductor wafer; the ultraviolet light generator emits ultraviolet light, and the grinding heat energy and ultraviolet light irradiation energy induce the decomposition of organic thermal decomposition and photodecomposition oxidants in the oil-based thermally active-photoactive composite grinding fluid to generate active free radicals, which undergo free radical oxidation reaction on the surface material of the wafer under mechanical stress, forming an oxide layer with low interfacial bonding strength.

[0009] The chemical mechanical grinding system proposed in this patent utilizes the low energy and large-area irradiation capabilities of ultraviolet light sources (the irradiation area can reach centimeters), combined with ultra-fine diamond abrasive wheels with interchangeable shapes and sizes, to simultaneously achieve flattening of semiconductor wafers and ultra-precision machining of complex curved and shaped surfaces of hard and brittle materials.

[0010] In a second aspect, the present invention provides a chemical mechanical grinding method for hard and brittle semiconductor wafers. The specific implementation steps of the grinding process are as follows:

[0011] S1: The second high-speed air-floating spindle drives the diamond abrasive grinding wheel to rotate, and the first high-speed air-floating spindle drives the wafer to rotate. The minimal lubrication device atomizes a certain amount of oil-based thermally active-photoactive composite grinding fluid into small droplets and sprays them onto the semiconductor wafer. At the same time, the first linear drive device and the second linear drive device apply the feed amount in the X direction and the grinding depth in the Z direction, and finally control the relative rotation of the diamond grinding wheel and the wafer for grinding.

[0012] S2. Adjust the ultraviolet light generator and use the grinding heat and ultraviolet radiation energy to induce the decomposition of organic thermal decomposition and photodecomposition oxidants in the oil-based thermally active-photoactive composite grinding fluid to produce active free radicals, which carry out free radical oxidation reaction on the surface material of the wafer under mechanical stress to form an oxide layer with low interface bonding strength; based on step S1, remove the oxide layer on the surface of the wafer by grinding.

[0013] As a further technical solution, the critical grinding depth d for mechanical removal of the oxide layer by diamond abrasives c The expression is as follows:

[0014] d c =λ(H / E) 1 / 2 (K c / H) 2

[0015] Where λ is the brittle-to-plastic transition factor of the hard-brittle semiconductor material, H is the hardness of the oxide layer on the wafer surface, E is the elastic modulus of the oxide layer on the wafer surface, and K c is the fracture toughness of the oxide layer on the wafer surface;

[0016] As a further technical solution, after step S2 is completed, the surface roughness S of the ground wafer can be measured using a white light interferometer. a The surface of the workpiece after grinding was observed using a field emission scanning electron microscope, and the damage type and scale of the wafer subsurface after grinding were observed using a focused ion beam-transmission electron microscope.

[0017] Preferably, the power of the ultraviolet light generator is 50-200W, the ultraviolet light wavelength is 100-400nm, the diamond abrasive grinding wheel is a metal bond or a ceramic bond diamond grinding wheel, and the diamond particle size is 0.5-10.0μm.

[0018] Preferably, the process parameters include workpiece rotation speed, grinding wheel rotation speed, workpiece feed speed, grinding wheel grinding depth and grinding fluid flow rate, the workpiece rotation speed is 100-2000rpm, the grinding wheel rotation speed is 500-3000rpm, the workpiece feed speed is 0.1-1mm / min, the grinding wheel grinding depth is 0.5-5μm, and the grinding fluid flow rate is 10-100mL / h.

[0019] Preferably, in the chemical mechanical grinding method for semiconductor wafers, the grinding heat and ultraviolet irradiation can induce the decomposition of thermally decomposable and photodecomposable organic oxidants in the oil-based thermally active-photoactive composite grinding fluid to produce active carbon radicals and carboxyl radicals through the following thermochemical and photochemical reactions:

[0020]

[0021] The chemical reaction equation for the formation of the wafer surface oxide layer is as follows:

[0022] M+HOCOCH2CH2C(CH3)(CN)·=HOCOCH2CH2C(CH3)(CN)M

[0023] M+·NC(CH3)(CN)CH2CH2COOH=MNC(CH3)(CN)CH2CH2COOH

[0024] M+εHOCH2CH2OC6H4CO·=HOCH2CH2OC6H4COM

[0025] M+·C(CH3)2OH=MC(CH3)2OH

[0026] Wherein, M is a hard and brittle semiconductor material.

[0027] Another aspect of the present invention provides an oil-based thermally active-photoactive composite grinding fluid for chemical mechanical grinding of hard and brittle semiconductor wafers. The steps for preparing the oil-based thermally active-photoactive composite grinding fluid are as follows:

[0028] S1, mixing polyethylene glycol, glycerol and vegetable oil and stirring to obtain a polyethylene glycol-glycerol mixed oil solution;

[0029] S2. According to the material composition and chemical bond type of the hard and brittle semiconductor material, the corresponding organic thermal decomposition type oxidant and organic photodecomposition type oxidant are selected to prepare a thermally active and photoactive oxidant complex, and the active-photoactive oxidant complex is added to a polyethylene glycol-propylene glycol mixed oil solution, and the complex is heated in a water bath and stirred to dissolve in the polyethylene glycol-propylene glycol mixed oil solution to obtain an oil-based thermally active-photoactive composite grinding fluid.

[0030] Preferably, the organic thermal decomposition type oxidant is azobiscyanovaleric acid (ACVA), azobisisobutyric acid (AIBME) or azobisisobutylamidine dihydrochloride (AIBA), the organic photodecomposition type oxidant is hydroxyacetophenone (2959), trimethylbenzoylphenylphosphonate (TPO) or trimethylbenzoyl (819), the thermally active-photoactive oxidant complex contains at least one thermally active oxidant and one photoactive oxidant or multiple thermally active and photoactive oxidants, the mass percentage of the thermally active oxidant to the photoactive oxidant is between 10:1 and 1:10, and the mass percentage of the thermally active-photoactive oxidant complex is between 1.0% and 10.0%.

[0031] Preferably, the mass percentage of the polyethylene glycol and glycerol is between 10% and 20%, which is used to adjust the viscosity and cooling performance of the grinding fluid.

[0032] Preferably, the vegetable oil is one or more of castor oil, olive oil, soybean oil, and rapeseed oil, which is used as a base liquid to dissolve organic thermally active and photoactive oxidants and adjust the lubrication properties of the grinding fluid. Considering that grinding generates a large amount of grinding heat, and excessive grinding heat can cause thermal damage to the workpiece and grinding wheel surfaces, the present invention provides an oily thermally active-photoactive composite grinding fluid using vegetable oil as a base liquid. During grinding, the organic thermally decomposable oxidant in the composite grinding fluid absorbs part of the grinding heat when it is cracked, which helps to suppress thermal damage to the workpiece and grinding wheel surfaces. At the same time, it generates a variety of activated carbon free radicals and carboxyl free radicals with the help of ultraviolet light energy and grinding heat energy, avoiding the problem of oxidation failure caused by the annihilation effect of a single free radical species. In addition, the lubrication properties provided by the base liquid vegetable oil can effectively reduce the wear of the ultrafine abrasive grinding wheel, ultimately improving the service life and processing efficiency of the grinding wheel.

[0033] Compared with the prior art, the present invention has the following advantages:

[0034] 1. The present invention utilizes an oil-based thermally active and photoactive composite grinding fluid to grind the surface material of semiconductor wafers under mechanical force. The grinding heat and ultraviolet radiation energy can controllable free radical oxidation, inducing the formation of an oxide layer with low interfacial bonding strength on the wafer surface, achieving high-efficiency removal of diamond abrasives with low load, thereby reducing or even eliminating the processing damage caused by high mechanical stress removal of single mechanical grinding. The wafer surface after chemical mechanical grinding is free of microcracks and breakage, and the workpiece surface roughness is S a The thickness of the workpiece subsurface crack is less than 2.0nm, and the damage layer thickness is less than 200nm. The present invention can simultaneously achieve high surface quality, low mechanical damage and high material removal rate ultra-precision grinding of hard and brittle semiconductor materials that are difficult to machine.

[0035] 2. The present invention utilizes a metal bond or a ceramic bond ultrafine diamond abrasive grinding wheel with high grinding efficiency and good surface accuracy. At the same time, the thermally active-photoactive composite grinding fluid oxidizes the surface material of the semiconductor wafer under the action of mechanical force, expands the controllable speed range of the free radical oxidation reaction, adapts to the mechanical force under different parameters, regulates the synergistic effect of chemical and mechanical grinding, effectively reduces the wear rate of the ultrafine diamond abrasive, increases the service life of the grinding wheel, reduces the number of grinding wheel dressing times, and can achieve high-efficiency ultraprecision grinding of hard, brittle and difficult-to-process semiconductor materials.

[0036] 3. Compared with the existing chemical mechanical grinding method or high-energy field assisted chemical mechanical method, the present invention utilizes the characteristics of grinding wheel grinding. By changing the size and shape of the ultrafine diamond abrasive grinding wheel, it can not only achieve high-efficiency ultra-precision flattening processing of semiconductor wafers, but also can be used for high-efficiency ultra-precision processing of complex curved and shaped surfaces of hard and brittle materials. At the same time, the vegetable oil-based thermally active-photoactive composite grinding fluid provided by the patent of the present invention can generate a variety of activated carbon free radicals and carboxyl free radicals with the help of grinding heat energy and ultraviolet light energy, avoiding the oxidation failure problem caused by the annihilation effect of a single free radical species, and the cracking of the organic thermal decomposition type oxidant can absorb part of the grinding heat, avoiding thermal damage to the workpiece and the grinding wheel surface. Finally, with the help of the lubricating properties provided by the base liquid vegetable oil, the wear of the ultra-fine abrasive grinding wheel can be effectively reduced, and the service life and processing efficiency of the grinding wheel can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Schematic diagram of the grinding system in the chemical mechanical grinding method of the present invention, 1-machine tool body, 2-left linear motor, 3-right linear motor, 4-left high-speed air-floating spindle, 5-right high-speed air-floating spindle, 6-vacuum adsorption fixture, 7-ultraviolet light generator, 8-grinding wheel fixture, 9-diamond abrasive grinding wheel, 10-semiconductor wafer, 11-oil-based thermally active-photoactive composite grinding droplets, 12-minimum lubrication device;

[0038] Figures 2(a), 2(b), and 2(c) are diagrams illustrating the process of chemical mechanical grinding of gallium nitride wafers using an oil-based thermally active-photoactive composite grinding fluid prepared in the chemical mechanical grinding method of the present invention and ultraviolet light irradiation chemical mechanical grinding;

[0039] Figures 3(a), 3(b), 3(c), 3(d), 3(e), and 3(f) are comparative diagrams of the surface morphology and roughness of gallium nitride wafers ground by a metal-bonded diamond grinding wheel using a vegetable oil-based fluid (grinding process A) and an oil-based thermally active-photoactive composite grinding fluid (grinding processes B and C) in the chemical mechanical grinding method of the present invention;

[0040] Figures 4(a) and 4(b) are TEM micrographs of the subsurface damage layer of a gallium nitride wafer ground by a metal-bonded diamond grinding wheel using a vegetable oil-based fluid (grinding process A) and an oil-based thermally active-photoactive composite grinding fluid (grinding process C) in the chemical mechanical grinding method of the present invention;

[0041] Figure 5 Process flow chart for the preparation of oil-based thermally active-photoactive composite grinding fluid in the chemical mechanical grinding method of the present invention and chemical mechanical grinding of hard and brittle semiconductor wafers. DETAILED DESCRIPTION

[0042] It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0043] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless otherwise clearly indicated in the present invention, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "include" and / or "comprising" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or their combinations;

[0044] This embodiment discloses a chemical mechanical grinding system and method for hard and brittle semiconductor wafers. The system utilizes a thermally active-photoactive grinding fluid to perform heat- and ultraviolet-controlled free radical oxidation on the semiconductor material, inducing oxidation of the semiconductor surface material to form an oxide layer with low hardness and low interfacial binding energy. Diamond abrasives then micro-cut with low loads to remove the oxide layer from the wafer surface, thereby improving the material removal rate while reducing processing damage caused by single high mechanical stress removal, thereby obtaining a semiconductor wafer with nearly no mechanical damage on the subsurface. This greatly reduces the processing time of subsequent chemical mechanical polishing, ultimately reducing the manufacturing and processing cost of the entire semiconductor wafer and accelerating the development of high-performance semiconductor devices.

[0045] The present invention is described in detail below with reference to the accompanying drawings:

[0046] like Figure 1As shown, the chemical mechanical grinding system for hard and brittle semiconductor wafers provided in this embodiment includes a machine tool body 1, on which a left linear motor 2 and a right linear motor 3 are arranged. The left linear motor 2 realizes the movement of the left high-speed air-floating spindle 4 in the X direction, and the right linear motor 3 realizes the movement of the right high-speed air-floating spindle 5 in the Z direction. A vacuum adsorption fixture 6 is installed on the left high-speed air-floating spindle 4, and the vacuum adsorption fixture 6 is used to fix the semiconductor wafer 10. The left high-speed air-floating spindle 4 drives the semiconductor wafer 10 to rotate; the right high-speed air-floating spindle 5 is installed with a grinding wheel fixture 8, and the grinding wheel fixture 8 fixes the diamond Abrasive grinding wheel, the right high-speed air-floating spindle 5 drives the diamond abrasive grinding wheel to rotate; an ultraviolet light generator 7 and a minimal lubrication device 12 are arranged on the outside of the semiconductor wafer; the minimal lubrication device 12 atomizes the oil-based thermally active-photoactive composite grinding fluid into small droplets according to a certain amount and then sprays them onto the semiconductor wafer; the ultraviolet light generator 7 emits ultraviolet light, and the grinding heat energy and ultraviolet light irradiation can induce the decomposition of organic thermal decomposition and photodecomposition oxidants in the oil-based thermally active-photoactive composite grinding fluid to produce active free radicals, which undergo free radical oxidation reaction on the surface material of the wafer under mechanical stress, forming an oxide layer with low interfacial bonding strength.

[0047] Based on the above system, this embodiment also provides a chemical mechanical grinding method for hard and brittle semiconductor wafers and a thermally active-photoactive composite grinding fluid. The following will be combined with the accompanying drawings to detail the technical content, objectives and effects of the present invention. Figure 5 As shown, a chemical mechanical grinding method for hard and brittle semiconductor wafers, the specific steps are as follows:

[0048] S1. Prepare oil-based thermally active and photoactive composite grinding fluid;

[0049] S2. Build a UV-diamond abrasive wheel chemical mechanical grinding system with UV irradiation function and adjustable and controllable irradiation emphasis and UV wavelength;

[0050] S3. Adjust the process parameters of chemical mechanical grinding and grind the oxide layer with low interfacial bonding force modified by the thermally active-photoactive composite grinding fluid on the surface of the wafer.

[0051] Specifically, the process parameters in step S3 include workpiece speed, grinding wheel speed, workpiece feed speed, grinding wheel grinding depth and grinding fluid flow rate, the workpiece speed is 100-2000 rpm, the grinding wheel speed is 500-3000 rpm, the workpiece feed speed is 0.1-1 mm / min, the grinding wheel grinding depth is 0.5-5 μm, and the grinding fluid flow rate is 10-100 mL / h.

[0052] The wafer surface obtained by this chemical mechanical grinding process has no micro cracks and breakage, and the surface roughness is S aLess than 2.0nm, no subsurface crack damage, and the thickness of the damaged layer is less than 200nm.

[0053] Furthermore, the specific operation steps of preparing the oil-based thermally active-photoactive composite grinding fluid in step S1 include:

[0054] S11, pouring polyethylene glycol, glycerol and vegetable oil into a beaker in sequence, and mechanically mixing and stirring for 5 to 10 minutes to obtain a polyethylene glycol-glycerol mixed oil solution;

[0055] S12. According to the material composition and chemical bond type of the hard and brittle semiconductor material, the corresponding organic thermal decomposition type oxidant and organic photodecomposition type oxidant are selected to prepare a thermally active and photoactive oxidant complex, and the thermally active-photoactive oxidant complex is added to the polyethylene glycol-propylene glycol mixed oil solution. The thermally active-photoactive oxidant complex is dissolved in the polyethylene glycol-propylene glycol mixed oil solution by heating and stirring in a water bath for 10 to 20 minutes to obtain an oil-based thermally active-photoactive composite grinding fluid, as shown in Figures 2(a), 2(b), and 2(c).

[0056] S13. According to the actual processing conditions, an oil-based thermally active-photoactive composite grinding fluid is selected to perform a free radical controlled oxidation reaction on the wafer surface material.

[0057] Preferably, the organic thermal decomposition type oxidant refers to an organic thermal decomposition type oxidant that is soluble in an oily solvent and decomposes to produce free radicals when heated to carry out a controlled free radical oxidation reaction on hard and brittle semiconductor materials under mechanical force, mainly including azobiscyanovaleric acid (ACVA), azobisisobutyric acid (AIBME) or azobisisobutylamidine dihydrochloride (AIBA); the organic photodecomposition type oxidant refers to an organic photodecomposition type oxidant that is soluble in an oily solvent and decomposes to produce free radicals after being irradiated by ultraviolet light to carry out a controlled free radical oxidation reaction on hard and brittle semiconductor materials under mechanical force, mainly including hydroxyacetophenone (2959), trimethylbenzoylphenylphosphonate (TPO) or trimethylbenzoyl (819). The thermally active-photoactive oxidant complex contains at least one thermally active oxidant and one or more photoactive oxidants, with the weight ratio of the thermally active oxidant to the photoactive oxidant ranging from 10:1 to 1:10, and the weight percentage of the thermally active-photoactive oxidant complex ranging from 1.0% to 10.0%. The weight percentages of polyethylene glycol and glycerol range from 10% to 20%. The base liquid vegetable oil is one or more of castor oil, olive oil, soybean oil, and rapeseed oil.

[0058] The main working principle of oil-based thermoactive-photoactive composite grinding fluid: Grinding heat and ultraviolet radiation can induce the decomposition of thermally decomposable and photodecomposable organic oxidants in the oil-based thermoactive-photoactive composite grinding fluid to produce active free radicals, which quickly undergo free radical oxidation reactions on hard and brittle semiconductor materials under mechanical force, promoting the formation of an oxide layer with low hardness and low interfacial bonding strength on the wafer surface; the diamond abrasive grinding wheel mechanically removes the oxide layer on the wafer surface, and the low-hardness and low-bonding-strength oxide layer breaks up and peels off in layers, and the rapidly exposed new surface is further oxidized and removed, achieving high processing efficiency; polyethylene glycol and glycerol are used to synergistically regulate the grinding heat in the grinding processing area, inhibit thermal damage to the wafer, and regulate the decomposition rate of the thermally decomposable oxidant; vegetable oil is used to regulate the contact stress between the diamond abrasive and the oxide layer, thereby achieving high-efficiency, low-damage ultra-precision grinding of difficult-to-process hard and brittle semiconductor materials.

[0059] Furthermore, the specific operation steps of the chemical mechanical grinding system constructed in step S2 include:

[0060] S21. Ultra-precision machine tools are placed in a room with yellow light without ultraviolet rays;

[0061] S22. A UV light generator with adjustable irradiation intensity and wavelength is installed inside the ultra-precision machine tool. The UV light generator is placed in the upper left of the wafer and grinding wheel grinding area through a multi-degree-of-freedom fixture, and is 2 to 5 cm away from the wafer to be processed;

[0062] S23. The semiconductor wafer to be processed is clamped onto a workbench connected to the left high-speed air bearing spindle by vacuum adsorption, and a diamond abrasive wheel with a micron or submicron grain size is fixed to the right high-speed air bearing spindle by a fixture;

[0063] S24, the oil-based thermally active-photoactive composite grinding fluid is atomized into small droplets at a certain speed by compressed air with the help of a minimal lubrication device and sprayed onto the surface of the wafer to be processed;

[0064] Specifically, the power of the ultraviolet light generator is between 50 and 200 W, the ultraviolet light band is between 100 and 400 nm, the diamond abrasive grinding wheel is a metal bond or a ceramic bond diamond grinding wheel, and the diamond particle size is between 0.5 and 10.0 μm, as shown in Figure 2(a), Figure 2(b), and Figure 2(c).

[0065] Furthermore, the specific operation steps of grinding the workpiece in step S3 include:

[0066] S31. Start the high-speed air-floating spindle on the right side of the machine tool to rotate the diamond abrasive grinding wheel, start the high-speed air-floating spindle on the left side of the machine tool to rotate the wafer, start the minimal lubrication device inside the machine tool to atomize a certain amount of oil-based thermal-active-photoactive composite grinding fluid into small droplets and spray them onto the semiconductor wafer, and simultaneously start the linear motors in the X and Z directions, then apply the feed in the X direction and the grinding depth in the Z direction, and finally control the diamond grinding wheel and the wafer to rotate relative to each other for grinding;

[0067] S32. Adjust the ultraviolet light generator and use the grinding heat energy and ultraviolet radiation energy to induce the decomposition of organic thermal decomposition type and photodecomposition type oxidants in the oil-based thermally active-photoactive composite grinding fluid to produce active free radicals, which carry out free radical oxidation reaction on the surface material of the wafer under mechanical stress, thereby forming an oxide layer with low interface bonding strength.

[0068] Specifically, the equations for the decomposition of the organic thermal decomposition type oxidant azobiscyanovaleric acid (ACVA) and the organic photodecomposition type oxidant hydroxyacetophenone (2959) by grinding heat energy and ultraviolet radiation energy are as follows:

[0069]

[0070] The chemical reaction equation for the formation of the wafer surface oxide layer is as follows:

[0071] M+HOCOCH2CH2C(CH3)(CN)·=HOCOCH2CH2C(CH3)(CN)M

[0072] M+·NC(CH3)(CN)CH2CH2COOH=MNC(CH3)(CN)CH2CH2COOH

[0073] M+HOCH2CH2OC6H4CO·=HOCH2CH2OC6H4COM

[0074] M+·C(CH3)2OH=MC(CH3)2OH

[0075] Wherein, M is a hard and brittle semiconductor material.

[0076] Based on step S3, the oxide layer on the surface of the wafer is removed by grinding. The critical grinding depth d for mechanical removal of the oxide layer by diamond abrasive is c The expression is as follows:

[0077] d c =λ(H / E) 1 / 2 (K c / H) 2

[0078] Where λ is the brittle-to-plastic transition factor of the hard-brittle semiconductor material, H is the hardness of the oxide layer on the wafer surface, E is the elastic modulus of the oxide layer on the wafer surface, and K c is the fracture toughness of the oxide layer on the wafer surface;

[0079] S33, using white light interferometer to measure the surface roughness S of the wafer after grinding a The surface of the workpiece after grinding was observed using a field emission scanning electron microscope, and the damage type and scale of the wafer subsurface after grinding were observed using a focused ion beam-transmission electron microscope.

[0080] The ultra-precision chemical mechanical grinding method for hard and brittle semiconductor wafers and the thermally active-photoactive composite grinding fluid of the present invention are further described with reference to the following examples:

[0081] In this embodiment, a 2-inch hard and brittle gallium nitride wafer is selected as the workpiece for processing. An ultra-precision machine tool designed and customized in the laboratory is used as the test platform. The ultraviolet light generator can stably irradiate the wafer to be processed and the thermally active-photoactive composite grinding droplets on the wafer surface. The metal bond diamond abrasive grinding wheel with an average particle size of 1 μm is used to grind the above wafer, as shown in Figures 2(a), 2(b), and 2(c). Chemical mechanical grinding is performed. After grinding, the sample is ultrasonically cleaned with alcohol for 10 minutes. The surface roughness S of the wafer after grinding is measured using a white light interferometer. a The measurement range is 50μm×50μm. The surface of the ground wafer is observed using a field emission scanning electron microscope, and the sub-surface of the ground wafer is observed using a transmission electron microscope combined with a focused ion beam sampling technique.

[0082] The specific implementation steps are as follows:

[0083] S1. Based on the chemical principle that organic thermal decomposition type oxidants interact with heat to produce free radical active species, the chemical principle that organic photodecomposition type oxidants interact with ultraviolet light to produce free radical active species, and the free radical reaction principle that free radical active species react with inert materials to form an oxide layer, and in view of the characteristics of hard and brittle semiconductor materials such as high hardness, great brittleness, and stable chemical properties, a thermally active-photoactive grinding fluid is configured that can produce free radical active species under the action of grinding heat energy and ultraviolet radiation energy and can carry out controllable free radical oxidation reactions on workpiece materials under mechanical stress:

[0084] S11. In a chemical experiment operation box, pour a certain amount of polyethylene glycol, glycerol, and vegetable oil into a beaker in sequence, and mechanically mix and stir for 5 to 10 minutes to obtain a polyethylene glycol-glycerol mixed oil solution;

[0085] S12, slowly adding a certain amount of an organic thermal decomposition type oxidant and a photodecomposition type oxidant complex to the polyethylene glycol-glycerol mixed oil solution, heating and stirring in a water bath for 10 to 20 minutes, dissolving the thermally active-photoactive oxidant complex in the polyethylene glycol-glycerol mixed oil solution, and obtaining an oil-based thermally active-photoactive composite grinding fluid;

[0086] S13. Pour the prepared thermally active-photoactive grinding fluid into a light-proof reagent bottle for later use.

[0087] S2. Build a UV-diamond abrasive wheel chemical mechanical grinding platform with UV irradiation function and adjustable and controllable irradiation emphasis and UV wavelength:

[0088] S21. Fix the UV light generator 3 cm from the contact interface between the wafer and the grinding wheel. The UV light generator can rotate and move to focus the UV light on a specified area.

[0089] S22. Clamp the wafer onto a workbench connected to the left high-speed air-floating spindle by vacuum adsorption, and fix a 1 μm-grit metal-bonded diamond abrasive wheel onto the right high-speed air-floating spindle.

[0090] S23. With the help of the micro-lubrication device inside the machine tool, the thermally active-photoactive composite grinding fluid configured in S1 is atomized into small droplets at a certain flow rate and sprayed onto the surface of the processed wafer.

[0091] S3. According to the composition, content and flow rate of the oxidant compound in the thermally active-photoactive composite grinding fluid obtained in S1, as well as the hardness, elastic modulus, fracture toughness and interface separation strength of the workpiece material oxide layer, the power and wavelength of the UV generator, the workpiece rotation speed, the grinding wheel rotation speed and the workpiece feed speed, the grinding wheel grinding depth and the grinding fluid flow rate are set. The process parameters are selected based on the mechanical properties of the wafer surface oxide layer, and the ultra-precision chemical mechanical grinding platform is started to perform the free radical oxidation reaction of the thermally active-photoactive composite grinding fluid of the hard and brittle semiconductor material, the dynamic removal of the surface oxide layer by diamond abrasive micro-cutting with small load and high efficiency:

[0092] S31. Start the high-speed air-floating spindle on the right side of the machine tool to rotate the diamond abrasive grinding wheel, start the high-speed air-floating spindle on the left side of the machine tool to rotate the wafer, start the minimal lubrication device inside the machine tool to atomize a certain amount of oil-based thermal-active-photoactive composite grinding fluid into small droplets and spray them onto the semiconductor wafer, and simultaneously start the linear motors in the X and Z directions, then apply the feed in the X direction and the grinding depth in the Z direction, and finally control the diamond grinding wheel and the wafer to rotate relative to each other for grinding;

[0093] S32. Turn on the ultraviolet light generator, adjust the power and band switch, and use the grinding heat energy and ultraviolet radiation energy to induce the decomposition of thermal decomposition type and photodecomposition type organic oxidants in the oil-based thermal-photoactive composite grinding fluid to generate active free radicals to carry out free radical oxidation reaction on the surface material of the wafer under mechanical stress, thereby forming an oxide layer with low interfacial bonding strength, and then carry out mechanochemical grinding of the workpiece material by free radical oxidation reaction of the thermal-photoactive composite grinding fluid and high-efficiency micro-cutting with diamond abrasive to mechanically remove the surface oxide layer with dynamic removal of the material;

[0094] After S33 and 50 to 60 minutes, the test machine is turned off and the wafer is removed. The wafer is then ultrasonically cleaned with alcohol for 10 minutes and the surface roughness S of the ground wafer is measured using a white light interferometer. a The measurement range is 50μm×50μm. The surface of the workpiece after grinding is observed using a field emission scanning electron microscope, and the damage type and scale of the wafer subsurface after grinding are observed using a focused ion beam-transmission electron microscope.

[0095] Furthermore, the present invention designed two grinding fluids and three sets of different grinding process tests to verify the advantages of the chemical mechanical grinding method based on the oil-based thermal-active and photoactive grinding fluid. The specific contents are as follows: Specific embodiment 1:

[0097] Grinding fluids: Grinding fluid A (30% polyethylene glycol, 10% glycerol, 60% castor oil) and Grinding fluid B (30% polyethylene glycol, 10% glycerol, 55% castor oil, 2% azobiscyanovaleric acid (ACVA), 3% hydroxyacetophenone (2959)).

[0098] Grinding process: Grinding process A (grinding fluid A, grinding fluid flow rate 30 mL / h, abrasive particle size 1 μm, workpiece speed 199 rpm, grinding wheel speed 2000 rpm, workpiece feed speed 0.5 mm / min, grinding wheel grinding depth 1 μm, grinding time 60 min, UV generator power 60 W, wavelength 365 nm), Grinding process B (grinding fluid B, grinding fluid flow rate 30 mL / h, abrasive particle size 1 μm, workpiece speed 199 rpm, grinding wheel speed 2 000 rpm, workpiece feed speed 0.5 mm / min, grinding wheel grinding depth 1 μm, grinding time 60 min, UV generator off) and grinding process C (grinding fluid B, grinding fluid flow rate 30 mL / h, abrasive particle size 1 μm, workpiece speed 199 rpm, grinding wheel speed 2000 rpm, workpiece feed speed 0.5 mm / min, grinding wheel grinding depth 1 μm, grinding time 60 min, UV generator on power 60 W, wavelength 365 nm).

[0099] The test results are shown in Figure 3(a), Figure 3(b), Figure 3(c), Figure 3(d), Figure 3(e), and Figure 3(f). The surface roughness S of the GaN wafer ground by grinding fluid A and grinding process A is a =9.4±0.7nm, and the wafer surface shows obvious grinding scratches and micro-cracks. Surface roughness S of the GaN wafer ground by grinding fluid B and grinding process B a =8.4±1.5nm, and the wafer surface shows obvious grinding scratches and micro-cracks. Surface roughness S of the GaN wafer ground by grinding fluid B and grinding process C a =1.5±0.7nm, with no obvious grinding scratches or micro-fractures on the wafer surface. Analysis of the subsurface damage layer in Figures 4(a) and 4(b) further demonstrates that UV-irradiated chemical mechanical grinding with an oil-based thermally active and photoactive composite grinding fluid (grinding process C) effectively inhibits the formation of subsurface microcracks on GaN wafers and reduces the thickness of the subsurface damage layer. In contrast, conventional vegetable oil mechanical grinding (grinding process A) exhibits subsurface microcracks and severe crystal slip, resulting in a damage layer thickness of up to 500nm.

[0100] By comparing the processing effects of mechanical grinding with a diamond abrasive wheel using a vegetable oil-based liquid and chemical mechanical grinding with an oil-based thermally active-photoactive composite grinding fluid, the three groups of experiments found that the chemical mechanical grinding method of the present invention was significantly better than the traditional mechanical grinding method. In particular, Experiment B used a thermally active-photoactive composite grinding fluid, but the ultraviolet light generator was not turned on during the processing. Only the thermal decomposition type oxidant in the grinding fluid was affected by the grinding heat energy, producing a single activated carbon free radical, so the oxidation effect was weak. However, in Experiment C, because the ultraviolet light generator was turned on, both the thermally decomposition type oxidant and the photodecomposition type oxidant were decomposed during the grinding process to produce a variety of activated carbon free radicals and carboxyl free radicals. The synergistic effect of each active free radical greatly increased the degree of oxidation of the surface material of the wafer, which was conducive to the formation of a thicker oxide layer on the wafer surface, thereby improving the removal of material and obtaining better grinding performance.

[0101] The above embodiments are merely descriptions of preferred implementations of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should fall within the scope of protection determined by the claims of the present invention.

Claims

1. A method for chemical mechanical grinding of hard and brittle semiconductor wafers by a chemical mechanical grinding system, characterized in that: The chemical mechanical grinding system includes: a machine tool body, on which a first linear drive device and a second linear drive device are provided, wherein the first linear drive device enables a first high-speed air-bearing spindle to move in the X direction, and the second linear drive device enables a second high-speed air-bearing spindle to move in the Z direction; an ultraviolet light generator and a minimal lubrication device are provided on the outer side of the semiconductor wafer; the ultraviolet light generator emits ultraviolet light; The method of the chemical mechanical grinding is as follows: S1: The second high-speed air-floating spindle drives the diamond abrasive grinding wheel to rotate, and the first high-speed air-floating spindle drives the wafer to rotate. The minimal lubrication device atomizes a certain amount of oil-based thermally active-photoactive composite grinding fluid into small droplets and sprays them onto the semiconductor wafer. At the same time, the first linear drive device and the second linear drive device apply the feed amount in the X direction and the grinding depth in the Z direction, and finally control the relative rotation of the diamond grinding wheel and the wafer for grinding. S2. Adjust the ultraviolet light generator to utilize the grinding heat and ultraviolet radiation energy to induce the decomposition of the organic thermal decomposition type oxidant and the organic photodecomposition type oxidant in the oil-based thermally active-photoactive composite grinding fluid to generate active free radicals, which carry out free radical oxidation reaction on the surface material of the wafer under mechanical stress, thereby forming a wafer surface oxide layer with low interface bonding strength; remove the wafer surface oxide layer by grinding on the basis of step S1; The mass ratio of the organic thermal decomposition type oxidant to the organic photodecomposition type oxidant is between 10:1 and 1:10; The power of the ultraviolet light generator is 50~200W; The grinding wheel speed is 500~3000 rpm, and the grinding fluid flow rate is 10~100 mL / h.

2. The method for chemical mechanical grinding of a hard and brittle semiconductor wafer according to claim 1, wherein: as follows: Critical grinding depth for mechanical removal of oxide layer from wafer surface using diamond abrasives d c The expression is as follows: d c = λ ( H / E ) 1 / 2 ( K c / H ) 2 in, λ is the brittle-to-ductile transition factor of hard-brittle semiconductor materials, H is the hardness of the oxide layer on the wafer surface, E is the elastic modulus of the oxide layer on the wafer surface, K c It is the fracture toughness of the oxide layer on the wafer surface.

3. The method for chemical mechanical grinding of a hard and brittle semiconductor wafer according to claim 1, wherein: as follows: After step S2 is completed, the surface roughness of the ground wafer is measured using a white light interferometer. S a The surface of the workpiece after grinding was observed using a field emission scanning electron microscope, and the damage type and scale of the wafer subsurface after grinding were observed using a focused ion beam-transmission electron microscope.

4. The method for chemical mechanical grinding of a hard and brittle semiconductor wafer according to claim 1, wherein: The ultraviolet light wavelength range is 100-400 nm. The diamond abrasive grinding wheel is a metal bond or a vitrified bond diamond grinding wheel. The diamond particle size of the diamond abrasive grinding wheel is 0.5-10.0 μm.

5. The method for chemical mechanical grinding of a hard and brittle semiconductor wafer according to claim 1, wherein: The rotation speed of the semiconductor wafer is 100~2000 rpm, the grinding wheel rotation speed is 500~3000 rpm, and the feed speed of the semiconductor wafer is 0.1~1 mm / min.

6. The method for chemical mechanical grinding of a hard and brittle semiconductor wafer according to claim 1, wherein: The grinding heat and ultraviolet radiation can induce the decomposition of the organic thermal decomposition type oxidant and the organic photodecomposition type oxidant in the oil-based thermally active-photoactive composite grinding fluid to produce thermochemical and photochemical reactions of active carbon free radicals and carboxyl free radicals as follows: The chemical reaction equation for the formation of the wafer surface oxide layer is as follows: Wherein, M is a hard and brittle semiconductor material.

7. The method for chemical mechanical grinding of a hard and brittle semiconductor wafer according to claim 1, wherein: The steps for preparing the oil-based thermally active-photoactive composite grinding fluid are as follows: S1, mixing polyethylene glycol, glycerol and vegetable oil and stirring to obtain a polyethylene glycol-glycerol mixed oil solution; S2. According to the material composition and chemical bond type of the hard and brittle semiconductor material, the corresponding organic thermal decomposition type oxidant and organic photodecomposition type oxidant are selected to prepare a thermally active-photoactive oxidant compound, and the thermally active-photoactive oxidant compound is added to a polyethylene glycol-propylene glycol mixed oil solution, and the compound is heated in a water bath and stirred to dissolve in the polyethylene glycol-propylene glycol mixed oil solution to obtain an oil-based thermally active-photoactive composite grinding fluid.

8. The method for chemical mechanical grinding of a hard and brittle semiconductor wafer according to claim 7, wherein: The organic thermal decomposition type oxidant is azobiscyanovaleric acid, azobisisobutyric acid or azobisisobutylamidine dihydrochloride; the organic photodecomposition type oxidant is hydroxyacetophenone, trimethylbenzoylphenylphosphonate or trimethylbenzoyl, The thermally active-photoactive oxidant compound contains at least one organic thermally decomposable oxidant and one organic photolyzable oxidant or a plurality of organic thermally decomposable oxidants and organic photolyzable oxidants, and the mass percentage of the thermally active-photoactive oxidant compound is between 1.0% and 10.0%.

9. The method for chemical mechanical grinding of a hard and brittle semiconductor wafer according to claim 7, wherein: The polyethylene glycol and glycerol are used to adjust the viscosity and cooling performance of the grinding fluid, with a mass percentage ranging from 10% to 20%; the vegetable oil is one or more of castor oil, olive oil, soybean oil, and rapeseed oil, which is used as a base fluid to dissolve organic thermal decomposition oxidants and organic photodecomposition oxidants, while also adjusting the lubrication performance of the grinding fluid.

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